You can add an event by just dragging on the calendar - it'll highlight the times you're dragging over, and then create the event. Alternately, you can type into the quick add box with phrases like "dinner next Saturday at 7 pm" and it'll figure out what that means and add an event. Inviting people is as easy as typing their email addresses, although oddly it doesn't auto-fill them using your gmail contact list (yet). I've heard gmail will also be getting calendar auto-integration soon - in other words, if someone sends you an email with a place, time, date, etc. it'll detect it and let you add that event to your calendar without having to type it by hand, in some sort of one-click goodness.
Of course, both gmail and google calendar integrate google maps as well; either will auto-detect locations and slap them in when appropriate.
http://calendar.google.com/
